Why?
It doesn't even look handy, as it blocks your "R" button, it blocks the blue On/Sleep indicator, you can't use the 2nd analog stick and the action buttons as fast as you can on PS2/PS3, X360, or GCN controllers, and most importantly, it makes your 3DS only bigger, so it no longer fits your pocket, plus the fact that the analog stick is going to be fucked up if it's not protected.
Besides, it'll make the access for smaller hands harder.